My first suspicion was that the reason for this was that the episodes were directed by different people, but Miguel Sapochnik directed them both. So there's really no excuse for it.
I agree the directing was poor, but in Episode 4 it's explicitly stated that about half each the Dothraki and Unsullied were lost, leaving ~20,000 Dothraki, ~4,000 Unsullied, and who knows how many northmen?